(1) Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire for passenger cars, small-size trucks and the like.
In particular, the invention aims to improve the rolling resistance, while being balanced with performance generally required in the tire of this type such as cornering stability, riding comfort against vibrations, wet skid resistance and the like, by combining the rationalization of radial profile of carcass with the optimization of properties of carcass cord, and principally belongs to the technical field relating to the internal reinforcement of the pneumatic tire.
(2) Description of the Prior Art
In order to reduce the rolling resistance of the tire, there have recently been proposed various countermeasures. Among them, there are primarily two countermeasures on the rubber material of the tread portion most remarkably contributing to the rolling resistance, one of which is the division of the tread rubber into two rubber portions in accordance with performances required for each portion, and the other of which is the reduction of abrasion loss of rubber at a ground contact area. However, such countermeasures can improve the rolling resistance up to a certain level, but if it is intended to attain improvement over the above level, they are obliged to sacrifice basic performances to be naturally required in the pneumatic tire, such as wet skid resistance, abrasion resistance and the like.
On the other hand, as the countermeasure from the viewpoint of the internal structure of the tire, it has been generally attempted to form the carcass line of the tire into an elliptical shape by reducing the aspect ratio of the tire, which is mainly intended to restrain the rolling resistance by reducing the shearing strain accompanied by the stepping-in and kicking-out actions at that region of the sidewall which contacts with the ground. In this case, the smaller the aspect ratio, the larger the shock produced in riding over undulations such as the road joints and the like during the running. Further, in order to equate the rolling radius of this low section tire to that of the standard size tire, it is necessary to adopt a rim with a size larger by one or two times than that of the usually used rim, which results in the increase of vehicle weight and occasionally offsets the effect of reducing the fuel consumption of the vehicle based on the reduction of the rolling resistance.